全文获取类型
收费全文 | 4381篇 |
免费 | 235篇 |
国内免费 | 282篇 |
专业分类
4898篇 |
出版年
2023年 | 92篇 |
2022年 | 82篇 |
2021年 | 118篇 |
2020年 | 112篇 |
2019年 | 157篇 |
2018年 | 130篇 |
2017年 | 139篇 |
2016年 | 117篇 |
2015年 | 117篇 |
2014年 | 185篇 |
2013年 | 337篇 |
2012年 | 149篇 |
2011年 | 211篇 |
2010年 | 145篇 |
2009年 | 245篇 |
2008年 | 211篇 |
2007年 | 267篇 |
2006年 | 218篇 |
2005年 | 214篇 |
2004年 | 215篇 |
2003年 | 154篇 |
2002年 | 125篇 |
2001年 | 82篇 |
2000年 | 88篇 |
1999年 | 77篇 |
1998年 | 89篇 |
1997年 | 62篇 |
1996年 | 53篇 |
1995年 | 64篇 |
1994年 | 49篇 |
1993年 | 44篇 |
1992年 | 51篇 |
1991年 | 47篇 |
1990年 | 28篇 |
1989年 | 25篇 |
1988年 | 20篇 |
1987年 | 30篇 |
1986年 | 26篇 |
1985年 | 37篇 |
1984年 | 40篇 |
1983年 | 31篇 |
1982年 | 31篇 |
1981年 | 29篇 |
1980年 | 21篇 |
1979年 | 29篇 |
1978年 | 22篇 |
1977年 | 23篇 |
1976年 | 14篇 |
1975年 | 14篇 |
1973年 | 11篇 |
排序方式: 共有4898条查询结果,搜索用时 0 毫秒
991.
A cell suspension culture, prepared fromPerilla frutescens var.crispa callus induced by Murashige and Skoog (1962) medium containing 2,4-dichlorophenoxyacetic acid (2,4-D, 1.0 ml/l) and kinetin (0.1 mg/l), contained caffeic acid derivatives as the phenolic components. Fresh and dry weights of the cells increased exponentially
for about 11 days after transfer to a fresh medium. The contents of caffeic acid and protein also reached a maximum on the
11th day, but α-amino nitrogen phenylalanine and tyrosine continued to increase in amount until the 20th to 23rd day. Caffeic
acid formation in the cells was increased by lowering the concentration of 2,4-D. The administration ofl-2-aminooxy-3-phenylpropionic acid (l-AOPP), 2-aminooxyacetic acid (AOA) andN-(phosphonomethyl)glycine (glyphosate) to the cells inhibited caffeic acid formation to a large extent. An 80% inhibition
of caffeic acid formation was caused by 10−4Ml-AOPP whereas phenylalanine and tyrosine contents of the cells became 7.5 and 2.3 times higher at thisl-AOPP concentration than those in the control. An 85% inhibition of caffeic acid formation was achieved at 10−3M glyphosate concentration, while 10−3M AOA inhibited caffeic acid formation by 95% and also growth rate by 80%. The influence of inhibitors on caffeic acid formation
is discussed in relation to the level of α-amino nitrogen, particularly aromatic amino acids, in the cell suspension cultures. 相似文献
992.
Inhibitory effects of Hymenaea and Copaifera leaf resins on the leaf fungus, Pestalotia subcuticularis 总被引:1,自引:0,他引:1
Leaf resins in the leguminous genera Hymenaea and Copaifera may play a role in restricting infection by the associated leaf fungus, Pestalotia. We tested this hypothesis by assessing growth of the geographically widespread Pestalotia subcuticularis presented with different compositions of resins. Leaf resins of Hymenaea and Copaifera are composed of the same suite of sesquiterpene hydrocarbons, but these vary quantitatively to form discrete compositional patterns among individuals and populations. Leaf resins may also contain sesquiterpene oxides; caryophyllene oxide, which may have been formed from the caryophyllene precursor common in these resins, inhibited fungal growth in vitro. 相似文献
993.
U-14C-phenylalanine and 3-14C-cinnamic acid were fed to detached Polygonum leaves through the cut petioles and to the bases of detached wheat leaves. After feeding, the leaves were divided into basal, middle and terminal segments; for each treatment of each plant more than 80% of the total radioactivity incorporated remained in the basal segment. The distribution of radioactivity between ethanol-soluble and insoluble fractions in each segment was examined. The basal segments contained more insoluble radioactivity than the terminal ones; the differences were far more marked for both plants when cinnamate rather than phenylalanine was administered. In view of the gross differences in distribution of radioactivity between the basal and terminal segments of each leaf, it is concluded that basal infusion of precursors is not the most suitable technique for studies of phenolic biosynthesis. 相似文献
994.
The absorption of sugars and organic acids by the daughter sporocysts of Microphallus similis (Jäg.)
The kinetics of uptake, accumulation against a concentration gradient, competitive inhibition and metabolic inhibition suggest that d-glucose enters the daughter sporocysts of Microphallus similis by active transport.Studies of competitive inhibition suggest that galactose, fructose, d-fucose, α-methyl-d-glucoside, d-mannose and 3-O-methylglucose are actively absorbed at the same locus as d-glucose. Glucosamine, d-ribose and l-fucose, however, may be actively transported at a different locus or may be taken up by simple diffusion.Acetate, pyruvate, citrate and succinate are absorbed by simple diffusion. 相似文献
995.
996.
997.
Summary Over a period of several years, quality characteristics of plant products have been studied intensively. Next to variety,
climate and location, the types and amounts of fertilizers used appeared to exert strong influences on quality parameters.
In recent research it could be shown that especially N-, but also P-, K- and Mg-supply affected dry matter content and furthermore
starch, crude protein, amino acids, nitrate, sugars and citric acid in cauliflower, lettuce, potato, spinach and tomato. Altogether
the results showed that the influence of variations in nutrient supply on the quality of important compounds in usable plant
parts is relatively high, but that different plants react not similarly to increasing or decreasing amounts of nutrients.
These relations are of special importance when the qualitative and nutritive values of plant products are evaluated for consumption
and processing purposes. 相似文献
998.
Summary The effect of spore concentration on spore germination and germtube growth ofTrichoderma hamatum on water agar and on potato dextrose agar (PDA) was studied. Increasing inoculum size up to 109 spores/plate on PDA and up to 107 spores/plate on water agar shortened the incubation period required for germtubes emergence and increased germination rate.
However, on water agar germination was inhibited at 108 and was completely arrested at 109 spores/plate. Inhibition in germination of 107 spores/plate was observed on water agar when the plates were preincubated with 109 spores/plate for 5 h or more. Addition of glucose and ammonium nitrate to the water agar medium allowed only 25% of the spores
to germinate at 109 as compared to 78% at 107 spores/plate after 8 h of incubation. Addition of polysaccharides to the C+N supplemented medium, significantly increased
germination up to 84% as compared to 100% on PDA, after 8 h of incubation. Germlings ofTrichoderma hamatum phialospores exhibited positive autotropism and anastamosis on both media. The phenomenon was positively related to inoculum
size, being most pronounced at 107 spores/plate. 相似文献
999.
Prephenate aminotransferase in Nicotiana silvestris Speg. et Comes is highly stable to thermal treatment. This property was exploited to obtain, by treatment at 70° C for 10 min, a residual level (1–4%) of aspartate aminotransferase activity that proved to be catalyzed exclusively by prephenate aminotransferase. The latter enzyme was the most mobile of all aspartate aminotransferase bands during polyacrylamide-gel electrophoresis conducted under non-denaturing conditions. This methodology for convenient assay of prephenate aminotransferase in crude extracts, as demonstrated for N. silvestris, may generally apply to higher plants since prephenate aminotransferase from a variety of plant sources has been found to exhibit high thermal stability.Abbreviations AGN
L-arogenate
- AT
aminotransferase
- ASP
L-aspartate
- GLU
L-glutamate
- HPP
4-hydroxyphenylpyruvate
- 2-KG
2-ketoglutarate
- OAA
oxaloacetate
- PPA
prephenate
- PPY
phenylpyruvate
Florida Agricultural Experiment Station, Journal Series No. 8286 相似文献
1000.
Effect of high and low sulfate concentrations on adenosine 5'-phosphosulfate sulfotransferase activity from Lemna minor 总被引:1,自引:0,他引:1
The effect of Na2 SO4 concentrations from 0 to 17.6 m M in the nutrient solution of Lemna minor L. strain 6580 on adenosine 5'-phosphosulfate sulfotransferase activity was examined. Routinely, the plants were cultivated on 0.88 mA SO4 2− . The enzyme activity was increased by 50 to 100% after transfer to 0 or 0.0088 m M SO4 2− . Transfer back to 0.88 m M rapidly decreased the enzyme activity to the initial level. Cultivation on 17.6 mM Na2 SO4 redueed extractable adenosine 5'-phosphosulfate sulfotransferase by 50%. The original level was rapidly re-established on 0,88 m M . In control experiments, a decrease in adenosine 5'-phosphosulfate sulfotransferase activity was also induced by K2 SO4 , whereas NaCl caused a small increase. This indicates that the observed effects are dependent on the sulfate ion. ATP-sulfurylase activity measured for comparison was only significantly affected by the omission of sulfate, which induced a 20% increase, indicating that this enzyme activity from Lemna minor is less suseeptible to changes in medium sulfate than adenosine 5'-phosphosulfate sulfotransferase. A close relationship between adenosine 5'-phosphosulfate sulfotransferase activity and the content of asparagine, glutamine, non-protein thiols and sulfate in the tissue was detected, indicating a positive control mechanism induced by amides and a negative mechanism induced by thiols and sulfate. 相似文献